Vacuum-type circuit breaker with force-supplementing means for increasing current-carrying abilities

ABSTRACT

Discloses a vacuum-type circuit breaker in which a wipe mechanism is interposed between the contact rod of the circuit breaker and the operating mechanism that supplies force for closing and opening the circuit breaker. The wipe mechanism comprises a wipe spring through which contact-closing force is transmitted from the operating mechanism to the contact rod during closing. Latching means comprising a cam mechanism operable when the contact rod is near closed position is provided for exerting a supplemental closing force on the contact rod that is transmitted to the contact rod via a force-transmitting path that effectively bypasses the wipe spring. During opening, the latching means is disabled by the operating mechanism before the wipe mechanism allows an effective opening force to be transmitted from the operating mechanism to the contact rod.

United States Patent [72] Inventors Willhln T. Slurp Phllodelphh; Claude H. Archer. Gradyville. both of. Pa. [21 1 Appl. No. 3,457 [22] Filed Jen. I6. 1970 [45] Patented Aug. 3, I97] [73] Assignee General Electric Company [54] VACUUM-TYPE CIRCUIT BREAKER WITH FORCE-SUPPLEMENTING MEANS FOR INCREASING CURRENT-CARRYING ABILITIES I 1 Claims, 3 Drawing Figs.

[52] US. Cl 200/! B, ZOO/I53 LB [51] Int. Cl. ..lI0lh33/66 I50] FieldolSeerelt.... ZOO/I44 B, I53 L. 153 LB, l69,148 F [56] References Cited UNITED STATES PATENTS 2,794,881 6/1957 Frank 335/l90 2,905,787 9/l959 l-leintz..... 335/73 3.161735 l2/l964 Miller........................... 200/144 8 3,209,l0l 9/l965 Peek etal..................... ZOO/144B 3,21 L868 l0/l965 Barkan et al ZOO/I48 B 3,300,609 1/1967 Flurscheim et al. ZOO/I44 B FOREIGN PATENTS $41,364 11193! Great Britain 200/144 B Primary Examiner- Robert S. Macon Attorneys-J. Wesley Haubner, William Freedman, Frank L.

Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT: Discloses a vacuum-type circuit breaker in which a wipe mechanism is interposed between the contact rod of the circuit breaker and the operating mechanism that supplies force for closing and opening the circuit breaker. The wipe mechanism comprises a wipe spring through which contact-closing force is transmitted from the operating mechanism to the contact rod during closing. Latching means comprising a cam mechanism operable when the contact rod is near closed position is provided for exerting a supplemental closing force on the contact rod that is transmitted to the contact rod via a force-transmitting path that effectively bypasses the wipe spring. During opening, the latching means is disabled by the operating mechanism before the wipe mechanism allows an efiective opening force to be transmitted from the operating mechanism to the contact rod.

OPERA TING MECHAN/SM PATENTED AUG 3 |97| SHEU 2 BF 2 uvvnvrons: WILL/AM TSHARP, CLAUDE H ARCHER,

41mm rib-um \IMIIIIIH ATTORNEY VACUUM-TYPE CIRCUIT BREAKER WITH FORCE- SUPPLEMENTING MEANS FOR INCREASING CURRENT- CARRYING ABILITIES This invention relates to an electric circuit breaker of the vacuum type, and, more particularly. relates to an electric circuit breaker of this type in which means is provided for applying a supplemental force to the contacts ofthe circuit breaker for holding the contacts closed against high currents and for assisting them in closing against high currents.

In the usual circuit breaker, the conductive path through the contacts of the circuit breaker constricts at the point of contact-engagement and thus contains portions ofa generally loop-shaped configuration adjacent the point of contact-engagement. Current flowing through each of these loop-shaped path portions produces magnetic forces tending to enlarge the loop, and these forces are in a direction tending to open the contacts of the breaker. These magnetic opening forces vary in magnitude in accordance with the square of the current flowing through the breaker, and hence during the highest current conditions, i.e., rated momentary currents and other short-circuit currents, extremely high magnetic contact-opening forces can be developed. A more detailed explanation of how these magnetic contact-opening forces are developed is contained in US. Pat. No. 3,225,160, Barkan, assigned to the assignee of the present invention. Note particularly FIG. 3 thereof.

If a circuit breaker is closed when a fault is present on the line, the above-described high magnetic opening forces are abruptly established near the end of a closing stroke. These forces tend to oppose the final portion of the closing stroke, and it is therefore necessary to provide high closing forces in order to overcome these opposing forces and complete the closing operation. In conventional circuit breakers, the necessity for providing these high forces for closing against shortcircuit currents is a major determinant of the size of the closing mechanism and the mechanism operator. The higher the closing force required, the larger and more powerful the closing mechanism and operator needed.

An object of our invention is to provide a circuit breaker which can be closed against maximum rated closing currents by a closing mechanism which, by itself, is insufficiently powerful to close the circuit breaker against such maximum rated currents.

In the usual closing mechanism of a vacuum circuit breaker there is a wipe spring which is compressed to allow the mechanism to move through a limited amount of overtravel after the contacts engage at the end of a closing stroke. It is desirable to keep this wipe spring as small as possible since the larger the spring, the greater the burden imposed on the closing mechanism by the need to compress the wipe spring during overtravel. But there are limitations on the extent to which the size of the wipe spring can be reduced since an important function of the wipe spring is to exert a hold-closed force on the contacts when the mechanism is in its fully closed position.

Another object of our invention is to provide a circuit breaker which is able to hold its contacts closed against the highest rated current, i.e., rated momentary current, despite reliance upon a wipe spring that, by itself, is not capable of providing a sufficiently high hold-closed force to prevent contact separation under the magnetic effect of such rated momentary current.

Another object is to provide latching means capable of exerting a supplemental hold-closed force on the contacts that is transmitted to the contacts via a force-transmitting path that effectively bypasses the wipe spring.

Still another object is to provide latching means of the above type capable of making available a supplemental holdclosed force for two separate interrupters and comprising a latching member common to the two interrupters that remains effective despite unequal contact erosion in the two interrupters.

In carrying out our invention in one form, we provide a vacuum-type circuit breaker that includes a vacuum interrupter comprising a contact rod for supporting its movable contact. For supplying closing force to the contact rod during a closing operation and opening force during an opening operation, an operating mechanism is provided which is coupled to the contact rod through a wipe mechanism. The wipe mechanism comprises a wipe spring through which contactclosing force is transmitted during closing. Latching means comprising a cam mechanism operable when the contact rod is in and near closed position is provided for exerting a supplemental closing force on the contact rod that is transmitted to the contact rod via a force-transmitting path that effectively bypasses the wipe spring. Means is provided for disabling the latching means early in an opening operation before an opening force is applied to the contact rod through the operating mechanism and wipe mechanism.

For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a sectional view, partly schematic, showing a vacuum circuit breaker embodying one form of our invention. The circuit breaker is shown in its fully closed position.

FIG. 2 is a view ofa portion of the circuit breaker of FIG. 1 at an intermediate point during an opening operation.

FIG. 3 is a view ofa portion of the circuit breaker of FIG. I in its fully open position.

Referring now to FIG. 1, there is shown a pair of vacuumtype circuit interrupters I0 that are electrically connected together in series-circuit relationship by a conductor schematically indicated at 8. Each interrupter 10 comprises a pair of separable contacts 11 and 12. Contact II is a stationary contact, and contact 12 is a movable contact that is vertically movable into and out of engagement with the stationary contact. In FIG. I each of the circuit interrupters is shown in its fully closed position where contact 12 engages contact 11. Opening of the upper interrupter is effected by driving contact 12 downwardly from its position of FIG. 1 into its fully open position of FIG. 3. Opening of the lower interrupter is effected by driving contact I2 upwardly from its position of FIG. I into its fully open position of FIG. 3. Closing of the interrupters is effected by driving movable contacts 12 back into engagement with their respective stationary contacts 11.

Each of the interrupters is a vacuum-type circuit interrupter that comprises a highly evacuated envelope I3 comprising a cylindrical insulating casing 14 and end caps 15 and [50 joined thereto by suitable vacuumtight seals. Stationary contact II is mounted on a stationary conductive rod 9 that is integrally joined to the outer end cap 150. Movable contact I2 is mounted on a movable conductive contact rod I7 that projects freely through the inner end cap 15. A flexible metallic bellows 171: permits vertical movement of rod I7 without impairing the vacuum inside envelope 13.

The two interrupters are mounted on a stationary frame 20 to which the inner end cap I5 of each interrupter is suitably attached. The axis of the two movable contact rods 17 are disposed in aligned, colinear relationship. For confining mo tion of these contact rods 17 to substantially straight line paths, each contact rod is provided with a guide link 22 and a suitable guide bearing 24 surrounding the contact rod. The guide link 22 is pivoted at one end on a stationary pivot 25 mounted on frame 20 and at its other end is pivotally connected by pin 26 to the movable contact rod.

For driving the movable contacts 12 between their fully open position of FIG. 3 and their fully closed position of FIG. I, a suitable operating mechanism is provided. Since this operating mechanism can be of any suitable conventional type, its major components have been represented by a block 30. An example of a suitable operating mechanism is shown and claimed in US. Pat. No. 3,] l3,l9l, Frank, assigned to the assignee of the present invention. In addition to the parts contained in block 30, the operating mechanism further com prises an operating rod 32 that is guided for straight-line movement in a horizontal direction by a bearing 34 on the stationary frame 20. Operating rod 32 has a slot 33 in its lefthand end that freely receives a stationary pin 64. Slot 33 per mits the operating rod to be moved longitudinally through a complete opening or closing stroke without interference from pin 64.

For transmitting closing motion from the operating rod 32 to the contact rod 17 of each interrupter, a connecting link 36 is provided for each interrupter. Each link 36 is pivotally connected by a pivot pin 35 at one end to operating rod 32 and at its opposite end is pivotally connected by pin 37 to a guide line 38, which, in turn, is pivotally mounted on stationary pivot 25.

For transmitting motion between each connecting link 36 and its associated contact rod 17, we provide a wipe mechanism 40 corresponding to the wipe mechanism shown and claimed in US. Pat. No. 3,l63,735, Miller, assigned to the assignee of the present invention. This wipe mechanism 40 comprises an intermediate link M of rod form that is pivotally connected at one end to contact rod 17 and is pivotally connected with lost motion at its opposite end to the connecting link 36. The connection to the contact rod is through pivot pin 26, and the connection to link 36 is through the previously described pivot pin 37, which is carried by links 36 and 38 and is journaled therein for rotation about its own axis. Pin 37 has a transversely extending hole therethrough which slidably receives the intermediate rod 41 to permit motion of pivot 37 along the length of rod 41. Disposed between pivot 37 and the other end of intermediate rod 4! is a compression spring 43 that encircles the intermediate rod 41. This compression spring 43 tends to urge the pivot 37 against a stop 45 fixed to rod 41. Since compression spring 43 is carried by rod 41, it will be apparent that the spring moves bodily with rod 4! during all interrupter operations.

When the operating rod 32 and link 36 are driven to the left from their position of FIG. 3 to initiate a circuit breaker closing operation, driving force is transmitted through each of the compression springs 43 to force the associated contact rod 17 in a closing direction. Following engagement of the contacts, the operating rod 32 and links 36 continue moving to the left, forcing each of the pivot pins 37 to slide along its intermediate rod 41, compressing the associated spring 43 and opening a space between the associated pivot pin 37 ans stop 45. This overtravel action during which spring 43 is compressed serves primarily to provide contact wipe. More specifically, this action assures that the contacts 12 and I] are firmly driven into engagement despite loss of contact material through wear and are erosion and without blocking the contacts of the other interrupter from engaging should the contacts of one interrupter engage ahead of the contacts of the other interrupter.

Opening of the circuit breaker is effected by driving operating rod 32 to the right from its closed position of FIG. 1 back to its fully open position of FIG. 3. During initial opening movement of the operating rod in this direction, no opening force is applied to the movable contact rod 17 since each of the pivot pins 37 is then merely sliding along its intermediate rod 41 without driving the intermediate rod. This lost motion continues until pivot pin 37 strikes its associated stop 45. When this occurs, an abrupt opening force is applied to the associated movable contact rod 17 to move it rapidly in an opening direction to separate contact 12 from contact ll, thereby interrupting the circuit.

When the circuit breaker is in its fully closed position of FIG. 1, each of the wipe springs 43 is exerting a hold-closed force on the contacts 12, ll of the associated interrupter. In a circuit breaker that has a high momentary current rating, this hold-closed force must be very high if it is to be able to hold the contacts closed against the high magnetic opening forces developed during the high momentary currents. It is possible to provide a wipe spring 43 heavy enough to provide the required high hold-closed force, but this is disadvantageous because an undue amount of force is needed to compress such a spring during the above-described travel. This necessitates an unduly large and powerful operating mechanismv To enable a lighter wipe spring to be relied upon, we provide force-supplementing latching means 60. This latching means 60 comprises a latch member 62 pivotally mounted on a stationary pivot pin 64 and a compression spring 66 urging the latch member in a clockwise direction about pivot 64. Latch member 62 has two cam surfaces 68 and 70 at its opposite ends. When the circuit breaker is in its closed position of FIG. 1, these cam surfaces 68 and 70 engage rollers 74 carried by extensions of the contact rods 17 and transmit a holdclosed force to the contact rods, as will soon be explained. The rollers 74 can be considered cam followers.

Cam surfaces 68 and 70 are able to transmit hold-closed forces to the contact I7 because they are offcenter surfaces that are divergent with respect to the periphery ofa reference circle 75 that has as its center the pivot axis of the latch member 62. In other woros, as the latch member 62 is pivoted clockwise about the axis of the pivot 64 by spring 66, the contact point between the followers 74 and the cam 68 or 70 becomes progressively further away from the pivot axis. The result is a wedging action that forces the two contact rods 17 toward their corresponding stationary contacts 11.

When the circuit breaker is in its fully open position of FIG. 3, latch member 62 is in a disabled condition. Spring 66 is urging the latch member clockwise, but inactive surfaces 76 and 77 on the latch member are engaging the followers 74 and no appreciable closing force is being exerted on the contact rods through the latch member. When the contact rods 17 are driven away from each other toward their closed positions, the followers 74 pass beyond the outer ends 80 and 82 of the inactive surfaces 76, 77. This enables spring 66 to pivot latch member 62 clockwise sufficiently to force the active cam surfaces 68 and 70 behind the followers 74. Further clockwise motion of latch member 62 produces the above-described wedging action which aids in driving the contact rods toward their fully closed position. The amount of force supplied through cam surfaces 68 and 70 to the contact rods 17 depends upon the size of latch-biasing spring 66 and the steepness of the cam surfaces 68 and 70 to the contact rods 17 depends upon the size of latchbiasing spring 66 and the steepness of the cam surfaces 68 and 70. The points 80 and 82 on the latch surfaces are so located that both the active cam surfaces 68 and 70 can begin to engage the followers 74 at a point well ahead of contact-engagement during closing. Thus, when the circuit through the contacts is completed, as by contact engagement or prestrike, the cam surfaces are exerting their wedging action on followers 74 and are capable of resisting any high magnetic opening forces developed as a result of circuit completion.

It will be noted that we use a single latch member 62 for supplying hold-closed force to both interrupters. The latch member may be thought of as a so id wedge between the two movable contact rods 17. It will be apparent that a portion of the magnetic opening forces developed at either interrupter are transmitted through latch member 62 to the contact rod of the other interrupter are transmitted through latch member 62 to the contact rod of the other interrupter as a force acting in a direction to hold closed the contacts of the other interrupter.

To permit the latch member 62 to adjust itself to compensate for unequal contact wear in the two interrupters, we provide the latch member with a slot 83 that slidably receives the stationary pivot 64 and enables the latch member to slide on the stationary pivot 64 along a reference line 84 extending between the two cam surfaces. By shifting slightly along reference line 84, the latch member can find a position where both of its cam surface 68 and 70 will always engage the followers 74 when the two interrupters are closed.

To prevent the latching means 60 from interfering with a desired opening operation, we provide means for disabling the latching means prior to transmission of an opening force to the contact rods. This disabling means comprises a latch-releasing member 85 carried by the left-hand end of the operating rod 32. When operating rod 32 is driven to the right during an opening operation, the tip of the latch-releasing member 85 engages latching member 62 and causes the latching member to pivot counterclockwise about stationary pivot 64. FIG. 2 shows the parts after such pivoting of latch member 62 has progressed to a point where the latch member would no longer be able to exert a substantial hold-closed force on the contact rods, even if engaged by follower rollers 74. In this regard, note that cam-starting points 80 and 82 have swung beyond the longitudinal axis 84 of the two contact rods.

At the point in the opening operation depicted in H0. 2, there is still lost motion 86 remaining in the wipe mechanism 40, and thus no opening force has yet been transmitted to contact rods 17. But after operating rod 32 has moved a slight additional distance to the right during the opening operation, the pivot pin 37 in the wipe mechanism strikes stop 45 and applies a sharply rising opening force to rod 4] and hence contact rod 17. This impact, together with further movement of operating rod 32 to the right, drives the movable contact rod 17 rapidly through its opening stroke. Since latch member 62 is then disabled, as seen in FIG. 2, it can exert no substantial closing force to interfere with continued opening at the desired high speed.

The latch-releasing means that effected the abovedescribed disabling of latch member 62 will now be described more specifically, It will be noted that latch-releasing member 85 is pivotally mounted on the operating rod 32 by a pivot pin 87 and is biased by a spring 88 counterclockwise about pivot 87 into contact with a stop 89 on the operating rod. When against stop 89, the latch releasing member 85 is an over center condition capable of transmitting thrust to latching member 62 when its tip 90 engages the latching member during the above-described latch-releasing operation. When, however, the latch-releasing member 85 is rotated slightly in a clockwise direction about 87, it is no longer capable of transmitting thrust to latch member 62. Such clockwise motion is effected by stationary pin 94 that engages an arm 92 of the latch-releasing member after the operating rod 32 has been moved through a portion of its opening stroke, as will soon be described.

After the contact rods 17 have been moved through a portion of their opening stroke to positions where the latching member 62, even if engaged by the follower 74, can exert no appreciable closing force on the followers, the releasing member 85 is effectively disabled so that it exerts no signifcant further releasing force on the latch member. This disabling action results from pin 94 engaging the arm 92 of the release member 85 to pivot it into its disabled position of FIG. 3.

A primary reason for disabling the latch-releasing member 85 is to prevent its interference with return of latch member 62 to its latching position at a proper point during a subsequent closing operation. in this regard, note that with the releasing member disabled, the latch spring 66 can bias the inactive surfaces 76 and 77 of the latch against the followers during the initial stages of the subsequent closing operation. Only these surfaces 76 and 77, and not the releasing member 85, are blocking the latch from returning to its latching position. During a subsequent closing operation, when closing has progressed to a point just prior to contact engagement, the followers pass beyond the points 80 and 82 and the latch spring 66 is then free to drive latch member 62 into its latching position without interference from the releasing member 85.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore. intend herein to cover all such chaiiges and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A vacuum-type circuit breaker comprising:

a. a vacuum interrupter comprising:

a,. a first contact and a second contact movable into and out of engagement with said first contact,

a.. an evacuated envelope in which said contacts are located,

a,. a contact rod supporting said second contact and extending through a wall of said envelope,

b, an operating mechanism through which contact-closing force is applied to said contact rod during a closing operation and through which contact-opening force is applied to said contact rod during an opening operation,

c. a wipe mechanism interposed between said contact rod and said operating mechanism and comprising a wipe spring through which said contact-closing force is transmitted from said operating mechanism to said contact rod during closing, said wipe spring yielding to permit overtravel of said operating mechanism after said contacts have engaged at the end of a closing operation,

d. latching means comprising a cam mechanism operable when said second contact is in and near its closed position for exerting a supplemental closing force on said contact rod that is transmitted to said contact rod via a forcetransmitting path that effectively bypasses said wipe spring,

e. and means controlled by said operating mechanism for disabling said latching means early in an opening operation before an opening force is applied to said contact rod through said operating mechanism, said wipe mechanism containing lost motion for providing a delay in the application of said opening force to said contact rod following initiation of an opening operation.

2. The circuit breaker ofclaim 1 in which:

said latching means and cam mechanism comprises a movable latch member and spring means for moving said latch member into a terminal position as said second contact moves into its fully engaged position,

b. said latch member has a cam surface, and said contact rod is coupled to a follower that is engaged by said cam surface when said second contact is in and near its fully engaged position,

c. said cam surface is shaped to exert wedging action on said follower as said latch member moves toward its terminal position, said wedging action developing a force on said follower acting in a contact-closing direction.

3. The circuit breaker of claim 1 in which said cam mechanism provides a wedging action that develops said supplemental closing force on said contact rod.

4. A vacuum-type circuit breaker comprising:

a. two vacuum interrupters each comprising:

a a first contact and a second contact movable into and out of engagement with said first contact,

a,. an evacuated envelope in which said contacts are located,

a,,. a contact rod supporting said second contact and extending through a wall of said envelope,

b. the contact rods of said interrupter projecting from their respective envelopes toward each other,

c. a common operating mechanism through which contactclosing force is applied to both of said contact rods during a closing operation and through which contact-opening force is applied to both of said contact rods during an opening operation,

d. Two wipe mechanism respectively interposed between said contact rods and said common operating mechanism and each comprising a wipe spring through which contact-closing force is transmitted from said operating mechanism to the contact rod of the associated interrupter during closing, each of said wipe springs yielding to permit overtravel of said operating mechanism after the contacts of the associated interrupter have engaged at the end of a closing operation,

e. latching means operable when both of said second contacts are near their closed positions for exerting supplemental closing force on both of said contact rods that is transmitted to said contact rods via force-transmitting paths that effectively bypass each of said wipe springs,

i. said latching means comprising a latch member common to the two interrupters through which a portion of the magnetic force tending to open one set of contacts is imparted to the other set of contacts as a force in a direction to hold said other set closed.

5. The vacuum circuit breaker of claim 4 in which the mounting of said common latch member allows the latch member to shift radially with respect to its pivot axis.

6. A vacuum-type circuit breaker as defined in claim 4 in which:

a. said common latch member is pivotally mounted and has cam surfaces at its opposite ends through which said supplemental closing force is transmitted to said contact rods,

b. said common latch member is so mounted that it can shift bodily in a direction radially of its original pivot axis to maintain both of said cam surfaces in engagement with their respective followers when the contacts of both interrupters are engaged despite unequal contact erosion, and

c. said latching means further comprises spring means biasing said latch member into a position where both of said cam surfaces engage said followers when the contacts for both interrupters are engaged.

7. The vacuum-type circuit breaker of claim 6 in which said common latch member has a slot therein receiving stationa ry pivot pin for said latch member, the slot allowing said latch member to shift with respect to said pivot pin in a direction along the length of said slot but blocking shifting transversely of said slot.

8. A vacuum-type circuit breaker as defined in claim 4 and further comprising: means controlled by said operating mechanism for disabling said latching means early in an opening operation before an opening force is applied to said contact rod through said operating mechanism, said wipe mechanism containing lost motion for providing a delay in the application of said opening force to said contact rod following initiation of an opening operation.

9. A vacuum-type circuit breaker as defined in claim 8 in which:

a. said means for disabling said latching means comprises releasing means responsive to an opening operation of said operating mechanism,

b. means is provided for disabling said releasing means at a predetermined point in said opening operation after said latching means has been disabled,

c. and means is provided for maintaining said releasing means disabled during a sufficient portion of said closing operation to allow said latching member to exert a closing force on said contact rod from an instant just preceding contact engagement during a subsequent closing [0. A vacuum-type circuit breaker as defined in claim 1 in which:

a. said means for disabling said latching means comprises releasing means responsive to an opening operation of said operating mechanism.

b. means is provided for disabling said releasing means at a predetermined point in said opening operation after said latching means has been disabled,

c. and means is provided for maintaining said releasing means disabled during a sufficient portion of said closing operation to allow said latching member to exert a closing force on said contact rod from an instant just preceding contact engagement during a subsequent closing.

1 l. The vacuum type circuit breaker of claim 10 in which:

a. said releasing means is maintained in a condition to trans mit thrust to said latching means when said contacts are engaged,

b. the disabling means for said releasing means renders said releasing means ineffective to transmit thrust to said latching means at said predetermined point in said opening operation. 

1. A vacuum-type circuit breaker comprising: a. a vacuum interrupter comprising: a1. a first contact and a second contact movable into and out of engagement with said first contact, a2. an evacuated envelope in which said contacts are located, a3. a contact rod supporting said second contact and extending through a wall of said envelope, b. an operating mechanism through which contact-closing force is applied to said contact rod during a closing operation and through which contact-opening force is applied to said contact rod during an opening operation, c. a wipe mechanism interposed between said contact rod and said operating mechanism and comprising a wipe spring through which said contact-closing force is transmitted from said operating mechanism to said contact rod during closing, said wipe spring yielding to permit overtravel of said operating mechanism after said contacts have engaged at the end of a closing operation, d. latching means comprising a cam mechanism operable when said second contact is in and near its closed position for exerting a supplemental closing force on said contact rod that is transmitted to said contact rod via a force-transmitting path that effectively bypasses said wipe spring, e. and means controlled by said operating mechanism for disabling said Latching means early in an opening operation before an opening force is applied to said contact rod through said operating mechanism, said wipe mechanism containing lost motion for providing a delay in the application of said opening force to said contact rod following initiation of an opening operation.
 2. The circuit breaker of claim 1 in which: a. said latching means and cam mechanism comprises a movable latch member and spring means for moving said latch member into a terminal position as said second contact moves into its fully engaged position, b. said latch member has a cam surface, and said contact rod is coupled to a follower that is engaged by said cam surface when said second contact is in and near its fully engaged position, c. said cam surface is shaped to exert a wedging action on said follower as said latch member moves toward its terminal position, said wedging action developing a force on said follower acting in a contact-closing direction.
 3. The circuit breaker of claim 1 in which said cam mechanism provides a wedging action that develops said supplemental closing force on said contact rod.
 4. A vacuum-type circuit breaker comprising: a. two vacuum interrupters each comprising: a1. a first contact and a second contact movable into and out of engagement with said first contact, a2. an evacuated envelope in which said contacts are located, a3. a contact rod supporting said second contact and extending through a wall of said envelope, b. the contact rods of said interrupter projecting from their respective envelopes toward each other, c. a common operating mechanism through which contact-closing force is applied to both of said contact rods during a closing operation and through which contact-opening force is applied to both of said contact rods during an opening operation, d. Two wipe mechanism respectively interposed between said contact rods and said common operating mechanism and each comprising a wipe spring through which contact-closing force is transmitted from said operating mechanism to the contact rod of the associated interrupter during closing, each of said wipe springs yielding to permit overtravel of said operating mechanism after the contacts of the associated interrupter have engaged at the end of a closing operation, e. latching means operable when both of said second contacts are near their closed positions for exerting supplemental closing force on both of said contact rods that is transmitted to said contact rods via force-transmitting paths that effectively bypass each of said wipe springs, f. said latching means comprising a latch member common to the two interrupters through which a portion of the magnetic force tending to open one set of contacts is imparted to the other set of contacts as a force in a direction to hold said other set closed.
 5. The vacuum circuit breaker of claim 4 in which the mounting of said common latch member allows the latch member to shift radially with respect to its pivot axis.
 6. A vacuum-type circuit breaker as defined in claim 4 in which: a. said common latch member is pivotally mounted and has cam surfaces at its opposite ends through which said supplemental closing force is transmitted to said contact rods, b. said common latch member is so mounted that it can shift bodily in a direction radially of its original pivot axis to maintain both of said cam surfaces in engagement with their respective followers when the contacts of both interrupters are engaged despite unequal contact erosion, and c. said latching means further comprises spring means biasing said latch member into a position where both of said cam surfaces engage said followers when the contacts for both interrupters are engaged.
 7. The vacuum-type circuit breaker of claim 6 in which said common latch member has a slot therein receiving a stationary pivot pin for said latch member, the slot allowing said latch member to shift witH respect to said pivot pin in a direction along the length of said slot but blocking shifting transversely of said slot.
 8. A vacuum-type circuit breaker as defined in claim 4 and further comprising: means controlled by said operating mechanism for disabling said latching means early in an opening operation before an opening force is applied to said contact rod through said operating mechanism, said wipe mechanism containing lost motion for providing a delay in the application of said opening force to said contact rod following initiation of an opening operation.
 9. A vacuum-type circuit breaker as defined in claim 8 in which: a. said means for disabling said latching means comprises releasing means responsive to an opening operation of said operating mechanism, b. means is provided for disabling said releasing means at a predetermined point in said opening operation after said latching means has been disabled, c. and means is provided for maintaining said releasing means disabled during a sufficient portion of said closing operation to allow said latching member to exert a closing force on said contact rod from an instant just preceding contact engagement during a subsequent closing.
 10. A vacuum-type circuit breaker as defined in claim 1 in which: a. said means for disabling said latching means comprises releasing means responsive to an opening operation of said operating mechanism, b. means is provided for disabling said releasing means at a predetermined point in said opening operation after said latching means has been disabled, c. and means is provided for maintaining said releasing means disabled during a sufficient portion of said closing operation to allow said latching member to exert a closing force on said contact rod from an instant just preceding contact engagement during a subsequent closing.
 11. The vacuum type circuit breaker of claim 10 in which: a. said releasing means is maintained in a condition to transmit thrust to said latching means when said contacts are engaged, b. the disabling means for said releasing means renders said releasing means ineffective to transmit thrust to said latching means at said predetermined point in said opening operation. 